Electrochemical detectors and voltammetric cells are known in the art and have been used with success for the analysis of flowing solution in the laboratory. Two-electrode and three-electrode cells are known. The three-electrode cell comprises a working electrode, a counter-electrode and a reference electrode which has the function of establishing and maintaining a constant potential relative to the working electrode or the sample solution. The sample solution is flown continuously through the cell. In principle, the electrodes may be affected by poisoning due to absorption with resulting passivation and loss of signal. In order to avoid such poisoning, the dropping mercury electrode has been adopted in many such cells.
U.S. Pat. No. 3,922,205 describes the basic structure of a voltammetric cell. U.S. Pat. No. 4,138,322 discloses a structure of shielded dropping mercury cathode. U.S. Pat. No. 4,260,467 describes a dropping mercury electrode which comprises a reservoir for liquid mercury, a mercury capillary at the outlet end of which mercury drops are formed, and a valve for selective air-purging passage of mercury from the reservoir to the inlet end of the capillary. An automated polarographic cell is described by C. N. Yarnitzky in Analytical Chemistry, Vol. 57, No. 9, August 1985, p. 2011-2015.
The efficiency of polarographic cells of the aforesaid type depends on the combination of a number of structural and functional features. A fully satisfactory combination, providing an industrially efficient such cell, has not been achieved so far in the art. The cells which are automatic and also on-line are expensive and not adequately efficient. In many cases, the prior art cells use a solid electrode which becomes polluted with time, so that the cell ceases to be reliable. In on-line, in-flow cells, the signal obtained is often proportional to the Reynolds number. Because of this, attempts have been made to design small cells, having high Reynolds number, comprising means for producing and controlling the dropping of the mercury electrode. Such means, however, being complicated and unreliable. Other cells are objectionable in that they require a very large volume of the sample solution, with resulting waste of time and chemicals.
A problem that is often encountered in the operation of voltammetric cells, both having drop mercury electrode and other electrodes, such as gold, platinum, glassy carbon etc., relates to their use for the analysis of sample solutions having an extremely low concentration. If the concentration falls to below 10 parts per billion, e.g., reaches the order of magnitude of tenths of parts per billion, a method known as stripping analysis must be used. This method involves passing an electric current through the static mercury drop electrode (SMDE) during one to three minutes so that the cationic material that is being analyzed accumulates in the static mercury drop, as would happen in normal electrolysis. Thereafter, the amount of material that has accumulated in the mercury drop is measured by measuring the current which passes between the mercury drop and the counter-electrode. Using this method requires extremely efficient mixing of the sample solution in the zone between the electrodes, which is difficult to achieve in small cells.
It is a purpose of this invention to provide an electroanalytical voltammetric cell, which is free from the drawbacks of the prior art cells.
It is another object of the invention to provide such a cell which is suitable for the analysis of sample solutions having an extremely low concentration.
It is a further object of the invention to provide such a cell in which a perfect mixing of the sample solution is obtained in the zone between the counter-electrode and the working electrode, particularly for the purpose of carrying out stripping analysis.
It is a still further purpose of the invention to provide all of the aforesaid features and advantages with a structure that is simple, reliable, inexpensive to make and durable in operation.
Other purposes and advantages of the invention will appear as the description proceeds.